#!/usr/bin/env python
# -*- coding:utf-8 -*-
# Author: G171117 JieJiSS

from math import pi as PI

def calc_area(r):
    r"""
    根据半径计算圆的面积
    """
    return PI * r ** 2

def f_in(prompt=""):
    r"""
    float(input(...))
    """
    return float(input(prompt))

print("圆的面积为：", calc_area(f_in("请输入圆的半径：")), sep='')
